The vent control mechanism is powered by engine vacuum. Start the engine and let it idle while you open the hood and listen for the tell-tale hissing sound of an open vacuum line. Once you find the loose hose/tube, it's just a matter of finding the orifice to which it belongs and plugging it back there. If the hose/tube is damaged, repairs might first need to be done before they can be inserted back and stay attached.
There are also vacuum lines controlling the vents behind the glove box, here is a link to a site that will help you with that side of the troubleshooting.
Under the hood in front of the transmission dip stick is a vacuum line. If it is disconnected or split it will make the air vents not work. If that's not it check your fuses.
I just fixed a similar problem on my 95 LeSabre, got info from this website. http://www.imcool.com/articles/aircondition/fix_vacuum_line.htm
Make sure refrigerant is fully charged. Does it have dual climate control? Perhaps passenger side sensor is not working properly
The vacuum supply line from the engine is disconnected or broken. When the vacuum supply is cut off the system defaults to the defrost mode.
There is a conduit with a fastener at the end. When I obtained my 2005 Buick LeSabre my rear air vents seemed to blow less than I had expected. After further inspection, I discovered that the conduit was not attached to the vent stack. So I played with it until I got it to slide into the vent stack and it seems to work as I would have expected at present.
The 1984 Buick uses vacuum lines to change air flow from defroster to ac vents. Check the entire vacuum system for leaks.
To change the CD player you need to disassemble the dash. First remove the 4 vents. There will be screws there. Also remove the woodgrain trim, there's screws there too. remove them and then pull the black dash piece out.
There may be something blocking the deflectors from sending air up there.
side floor vents are manually closed, been along time but i think that's right.
If the heat selector will not switch from defrost to the dash or foot vents on a 1992 Buick, it is possible that the cable connected to the vent is broken. When the selector is moved from defrost, a cable opens the other selected vents on the heater.
Open, so the moisture can escape.